Thermal Field And Flow Behavior Research Of Selective Laser Melting

Additive manufacturing technology has changed the traditional manufacturing method.It has strong processing applicability for parts with complex geometric features,and has broad application prospects in aviation,aerospace,automotive and other fields.Selective laser melting(SLM),as one important technololgy of additive manufacturing,has the advantages of high precision,high material utilization,low pollution,etc.However,the problems,such as pore defects and balling effects caused by unstable flow of the molten pool and the residual stress caused by dramatically changing temperature field,restrict the wide application of this technology.Therefore,understanding the pore defect,the balling effect mechanism and accurately obtaining the temperature field information are of importance for the development of SLM.In this paper,a powder spreading model is established by the discrete element method.A hydrodynamic model of the molten pool and a temperature field model are established by the finite volume method.The multi-scale modeling method is used to simulate the laser melting process in SLM.The powder distribution after spreading on different surface conditions is studied.The average coordination number and packing density are used to characterize the density of the powder bed.The simulation results of the powder spreading model are introduced into the hydrodynamic model of the molten pool to predict the flow behavior.The packing density is used to solve the thermal conductivity of the powder layer.The influence of laser power on the temperature history is studied.The surface morphology of SLMed samples observed by laser confocal microscopy and ultra-depth of field microscope.The temperature history of the SLM process measured by infrared camera are used to verify the reliability of the multi-scale model.Through the theoretical and experimental analysis of the flow behavior of the molten pool,the relationship between the flow behavior of the molten pool and the lack of fusion pores and balling effect is established.The results show that the powder on the as-built surface has a denser powder distribution than the flat surface,but the molten pool on the as-built surface has unstable flow and is susceptible to balling effect;the lack of fusion holes between the tracks are mostly located at the necking area of the track.This research has guiding significance for the multi-scale modeling method of SLM and the study of pore defect,balling effect and temperature history and distribution prediction.